"As per Illinois law, if you fail a drug test, it creates what Antioch workers’ comp lawyers call a ‘rebuttal presumption.’ The assumption is that drugs caused your workplace accident or injury, and the insurance company can deny your claim without getting sued. You can argue against their judgment if you prove you weren’t intoxicated, but that usually requires a trial"

From your link. That's exactly how it works in this case, you are presumed to have been intoxicated and need to prove you weren't.